Alderweireld only made the move last summer to Atletico from Ajax, where he won three Eredivisie titles and one Dutch Cup.

He told the club's official website: "The trainer [Ronald Koeman] likes how I play and really wanted me to come here, which is what convinced me to come. I really wanted a trainer that believes in me, because that's important.

"I think this is an upcoming club. Southampton put a lot of effort in to get me, so now I want to give something back to the club by doing my best to help the team. I want to give everything for Southampton."

Southampton have also confirmed the signing of Senegalese winger Sadio Mane from Red Bull Salzburg for an undisclosed fee.

Koeman told the Saints' website: "He can play in different positions in the attack -- he can play on the left or right side and also as a number nine.

"It is incredible how many goals he has scored from his position, and I hope that he will do the same for us."

Alderweireld and Mane are the seventh and eighth new additions at St Mary's this summer following the arrivals of Dusan Tadic, Graziano Pelle, Fraser Forster, Shane Long, Florin Gardos and Ryan Bertrand.